A copper surface is apt to be readily oxidized. When during a process a copper surface is allowed to stand in the air for a long time or comes into contact with an oxidizing atmosphere, the copper surface is oxidized and it is necessary to perform an intermediate treatment, such as pickling, during a process thereafter. In order to prevent the oxidation of a copper surface, it has hitherto been a general practice to clean the copper surface at the beginning of a process, and to continuously perform the process thereafter in a clean atmosphere, thereby preventing the oxidation of the copper surface. However, in a case where a process is interrupted in the course of the process and a copper surface has to be exposed to the air for a long time or in a case where a copper surface has to be exposed to an oxidizing atmosphere in the course of the process, it has often been necessary to perform the re-cleaning of the copper surface in the course of the process.
Ideas to solve this problem have been proposed. For example, Patent Document 1 relates to a method of manufacturing a semiconductor integrated circuit device having a buried interconnect in which copper is a main conductive layer, and in an ammonia plasma treatment step, which is one of the processes adopted in this method, a thin nitrided layer is formed on a surface of copper and it is suggested that the formation of an oxide layer is capable of being suppressed because of this nitrided layer. Furthermore, several techniques using plasma have been proposed. However, the case of the techniques using plasma poses the problem that it is impossible to deny the possibility that what is called plasma damage occurs.
On the other hand, in recent years, metal nanoparticles have been developed. Metal nanoparticles refer to particles of not more than 100 nm, preferably not more than 30 nm in terms of primary average particle diameter, can be prepared by an in-gas evaporation method (a method by which metals and the like are evaporated in an inert gas and nanosize-particles having uniform particle size are manufactured) and can be dispersed in an organic solvent, such as toluene. In order to stabilize dispersibility for a long period, it is effective to add a dispersant, an anti-foaming agent and the like and by adding a thermosetting resin, such as phenol resins and epoxy resins, it is effective to accelerate the coalescence and fusion among nanoparticles by the setting and contraction of the thermosetting resin. Examples of materials include copper, silver, gold and the like. Fine particles of these materials have the great feature that they permit direct writing by an ink jet technique. Metal nanoparticles are contained in an organic solvent, and a desired pattern of metal nanoparticles is written by an ink jet technique which has been put into practical use in a printer.
Although noble metals such as silver and gold are inherently difficult to oxidize, copper has the property of being easy to oxidize compared to silver and gold. After the writing of an interconnect pattern, it is necessary to perform a heat treatment (150 to 300° C. or so) for evaporating the organic solvent and furthermore for causing copper particles to adhere to each other. However, the copper surface is oxidized also during this heat treatment. In metal nanoparticles, the proportion of atoms in the surface portion is large and, therefore, metal nanoparticles have the problem that interconnect resistance increases due to the formation of surface copper oxides.
Because an organic solvent cannot be thoroughly removed only with heat treatment, the resistivity of a copper interconnect cannot be reduced and under the present circumstances copper cannot be used as interconnects. With respect to a reduction of resistance after writing, in particular, in the case of copper, adequate solutions have not been found out as yet.
Also, instead of direct writing techniques such as an ink jet technique, in techniques using lithography which involves mixing copper in a resist, various ideas to reduce the resistance of copper have been proposed and, for example, Patent Document 6 is known. In a reduction heat treatment technique used in Patent Document 6, the reduction heat treatment is performed in an inert gas (or in a vacuum) containing not more than 4% of molecular (H2) hydrogen at temperatures of 200 to 450° C. Thus, in this technique, a direct writing technique is not adopted although the fine particles of copper are used, and the reduction temperature is as high as 200 to 450° C. At such high temperatures, it is difficult to use this technique in the semiconductor mounting region.    [Patent Document 1]: Japanese Patent Laid-Open No. 2002-110679    [Patent Document 2]: Japanese Patent Laid-Open No. 2003-347241    [Patent Document 3]: Japanese Patent Laid-Open No. 2001-176878    [Patent Document 4]: Japanese Patent Laid-Open No. 2004-127503    [Patent Document 5]: Japanese Patent Laid-Open No. 11-26465    [Patent Document 6]: Japanese Patent Laid-Open No. 2002-75999